1. Field of the Invention
This invention pertains generally to magnetic pole apparatus and is more particularly concerned with dynamoelectric machines provided with what are known as interpoles between the main poles.
2. Description of the Prior Art
In the practice of electrical engineering, it is well known that sparking between the brushes and the commutator of dynamoelectric machines is a problem that can result in reduced life of the brushes and the commutator. Sparking may arise from either or both the effects of armature reaction and self-inductance of armature coils undergoing commutation. To the end of eliminating or reducing this nuisance to the extent practically possible in order to improve brush and commutator life, it has become the practice to employ commutating poles or interpoles disposed between the main field poles of high output direct current machines. Pole face or compensating windings may also be employed if the particular interpole design does not eliminate armature reaction to the extent desired.
The tips of the interpole laminations are shaped in such a way as to give a corrective magnetic flux to counteract the induced voltages in the commutated armature coils as they move through the commutating zone. Calculations of the shape of the pole tip and magnetic strength of the interpole flux are usually based on a linear commutation. That means it is assumed that the current in the coils undergoing commutation changes linearly from the leading edge of the brush from one polarity to the other polarity on the trailing edge of the brush. Because of the symmetry of the assumed current change, no influence of the main field magnetic circuit on the magnetomotive force, or ampere-turns.
In actual operation of symmetric interpoles with assumed linear commutation, sparking nevertheless will occur. In order to get sparkless commutation, the interpole gaps are made smaller by inserting magnetic shims between the poles and the yoke or the frame to give overcompensation, which forces the armature coils to commutate early. However, early commutation upsets the symmetry of the current distribution under the brushes and gives a demagnetizing effect on the main pole windings, which weakens the main pole flux and, especially at weak field, gives a rising speed characteristic with increasing load.
It has been known for a long time that a symmetrically shaped interpoles on machines with a single direction of rotation with a smaller air gap on the trailing edges of the poles will give better commutation and less influence on the main pole strength. This type of interpole air gap gives, in effect, a shift of the centerline of the flux in the direction of rotation with forced commutation at the second half of the interpole, thus restoring linear commutation without the sparking at the trailing edges. Unfortunately, the machine is confined to a single direction of rotation, hence the principle is useless for application where bi-directionality of the machine is a necessity, as for example in vehicular, hoist, and metal-mill applications.